1. Field of the Invention
The present invention relates to a freeze-thawing tank for use in freeze-thawing an aqueous dispersion containing colloidally stabilized solid particles such as, for example, polymer lateces or water-insoluble polysaccharides, or sludge.
2. Description of the Prior Art
For accomplishing a liquid-solid separation of an aqueous dispersion containing colloidally stabilized particles into a water component and a solid component, there is known a chemical flocculating process in which the solid particles are flocculated in the presence of a salt such as common salt or calcium chloride, an acid such as, for example, hydrochloric acid or acetic acid, a hydrophilic solvent such as, for example, alcohol or acetone, or a water-soluble polymer such as, for example, polyacrylamide or polyethylene-imine, and a physical flocculating process including a spray drying method and a freeze-thawing method.
The freeze-thawing method is known as a liquid-solid separation technique in which a fluid medium to be treated is frozen and then thawed so that the colloidally stabilized state of solid particles contained in the fluid medium can be lost to facilitate flocculation of the solid particles. This freeze-thawing method is extremely effective for the liquid-solid separation of functionally modified polymer lateces which, when the chemical flocculating method or the spray drying method is employed, would result in a chemical reaction.
In the practice of the freeze-thawing method, a freeze-thawing tank is generally employed, an example of which is shown in FIGS. 9(a) and 9(b). The prior art freeze-thawing tank comprises a generally funnel-shaped tank body 61 having a plurality of, for example, two, spirally coiled tubes 71 and 72 accommodated coaxially within the tank body 61. This prior art freeze-thawing tank is so designed and so structured that, when in use, the fluid medium to be treated is filled into the tank body 61, cooled brine is allowed to flow through the coiled tubes 71 and 72 to freeze the fluid medium and, after the fluid medium has completely been frozen, heated brine is then allowed to flow through the coiled tubes 71 and 72 to thaw the fluid medium before it is discharged to the outside of the tank body 61.
It has however, been found that the prior art freeze-thawing tank of the construction shown in FIG. 9 has a problem in that, since a space A between each neighboring convolution 71a of any one of the coiled tubes 71 to 72, as shown in FIG. 10(a), constitutes a dead space, solid particles contained in the fluid medium being treated tend to build up in the spaces A and, therefore, the fluid medium, after having been treated cannot be completely discharged out of the freeze-thawing tank. The prior art freeze-thawing tank also has a problem in that, while each of the coiled tubes 71 and 72 are formed by spirally winding a straight tube, not only is the spiral winding of the straight tube very difficult to achieve, but a mounting of the whole number of the coiled tubes within the tank body is also difficult to achieve. In any event, the prior art freeze-thawing tank is complicated in structure and requires a complicated and time-consuming workability in cleaning and maintenance.
Moreover, according to the structure shown in FIG. 9, the distance between neighboring cooling faces of any one of the coiled tubes is not constant and, consequently, the width of a portion of the fluid medium to be treated which is present between the neighboring cooling faces of any one of the coiled tubes varies depending on the position. Accordingly, as shown in FIG. 10(b), at the time portions of the fluid medium present within regions F spaced a predetermined distance from a longitudinal axis of each of the convolutions 71a and 72a of any one of the coiled tubes have been completely frozen, portions of the fluid medium present in regions f have not yet been frozen completely. Accordingly, in order for the entire quantity of the fluid medium within the tank to be completely frozen, those portions of the fluid medium present in the regions F has to be supercooled so that the portions of the fluid medium present in the regions f can be cooled to freeze in contact with the supercooled portions of the fluid medium, requiring a prolonged length of time to complete the complete freezing of the fluid medium as a whole. The reverse of the foregoing description may equally apply to the case where the frozen fluid medium is to be thawed.
In addition, a freezing condition or state varies between the regions F and the regions f and, therefore, the solid particles, after having been thawed, may exhibit a varying particle size. Accordingly, where the solid particles extracted from the fluid medium by the utilization of the freeze-thawing technique are desired to be employed for a secondary purpose, the varying particle size may pose a problem.